1. Field of the Invention
The present invention relates to power supply devices that produce a desired output voltage from an input voltage, and to electronic appliances provided therewith.
2. Description of Related Art
Conventionally, as one of the stabilized power supplies that suffer less from heat loss and operate with a relatively high degree of efficiency if there is a great difference between the input and output, a switching regulator that produces a desired output voltage from an input voltage by driving an energy storage element (such as a capacitor or a coil) by performing on/off control (duty control) of an output transistor is widely used.
A conventional common switching regulator is provided with an error amplifier that amplifies a difference between a feedback voltage Vfb varying with an output voltage Vout and a predetermined reference voltage Vref, and is configured so as to perform on/off control of an output transistor by using an output signal (error voltage Verr) of the error amplifier. More specifically, such a switching regulator is configured so as to produce a PWM (pulse width modulation) signal having a duty ratio commensurate with a comparison result between the error voltage Verr and a predetermined slope voltage Vslp (triangular or ramp wave) and control on/off of the output transistor by using the PWM signal thus produced.
In addition, the conventional switching regulator is provided with a soft-start circuit as a means of preventing an inrush current at startup. More specifically, the soft-start circuit is configured so as to produce a soft-start voltage Vss that starts to rise gradually as an enable signal EN (operation enable signal) rises, and a PWM comparator is configured so as to compare either the error voltage Verr or the soft-start voltage Vss, whichever is lower, with the slope voltage Vslp, and thereby produce a PWM signal having a duty ratio commensurate with the comparison result.
An example of a conventional technology related to what has been described thus far is seen in JP-A-H7-336999 (hereinafter, “Patent Document 1”), which the applicant of the present invention once filed.
Certainly, with the conventional switching regulator described above, it is possible to prevent an inrush current at startup by using the soft-start circuit.
However, the conventional switching regulator has the following drawback. In this switching regulator, a coil current IL is reduced by gradually increasing the on-duty ratio of the output transistor. Although this helps prevent an inrush current at startup, the rise of the output voltage Vout and hence the response speed of output feedback control is slowed down.
In particular, LED drivers that adjust the brightness (the average LED current value) of LEDs (light-emitting diodes) by driving the enable signal EN with PWM (pulse width modulation) suffer a reduction in accuracy of brightness adjustment with a reduction in response speed of the above-described output feedback control.
It is true that disabling soft-start enhances the response speed of the output feedback control. However, as indicated by symbol (X) in FIG. 12, this results in the occurrence of an inrush current in the coil current IL every time the enable signal EN rises, raising the possibility of breakdown of the coil or the output transistor.
In addition, as indicated by symbol (Y) in FIG. 12, disabling soft-start to enhance the response speed makes the rising speed of the feedback voltage Vfb depend heavily on variations in the input voltage Vin (in this figure, a solid line indicates the behavior observed when the input voltage Vin is high, and a broken line indicates that observed when the input voltage Vin is low).
In particular, in the aforementioned LED drivers, when the duty ratio of the enable signal EN is small (when the brightness is set low), the rising speed of the feedback voltage Vfb varies greatly depending on variations in the input voltage Vin. This results in variations in LED current (=Vfb/R), which may be visually recognized by the user as flicker.
Incidentally, the cause of the dependence of the rising speed of the feedback voltage Vfb on input voltage is as follows. Since the input voltage Vin varies, whereas a limit value ILmax of the coil current IL is kept constant, the input power (=ILmax×Vin) of the switching regulator varies with the variations in input voltage Vin, resulting in variations in rising speed of the output voltage Vout (and hence feedback voltage Vfb).